Switching Off the Surface Conductivity of Strontium Titanate by Non-Volatile Organic Adsorbates

Strontium titanate is a well-studied perovskite oxide. Due to its interesting properties and because of the possibilities of interfacing it with other oxide materials, e.g., with lanthanum aluminate resulting in a high-mobility 2D electron gas, it has generated a lot of interest.

In the present study an international team based at the Swiss Light Source at PSI, at ELETTRA and at the MAX IV laboratory, was able to demonstrate that the electrical conductivity at the surface of strontium titanate can be tailored by depositing the prototypical organic acceptor molecule tetracyanoquinodimethane (TCNQ) onto it. Already tiny amounts as low as 20% of a monolayer coverage are sufficient to completely switch off the surface conductivity. The conductivity switching is explained by the charge transfer from the strontium titanate surface to the TCNQ molecules, which become negatively charged. Subsequently, the band bending in the strontium titanate is lowered and the carriers are depleted.

Furthermore, the authors found that geometry of the oxygen octahedra surrounding the titanium atoms is modified upon TCNQ adsorption - from elongated toward compressed geometry. This understanding is crucial in order to work toward functional oxide-organic mixed heterostructures with potential applications in electronic  or photovoltaic devices.